Subreddit names and user names are limited to 20 characters (OK, so for subs it's actually 21...) Submit subs that amusingly hit the limit (perhaps by leaving out a character), posts that try to link to a sub that is waaaaaaaaayyyyyyyyyy too long, or other examples (usernames, road signs, etc) of amusing or interesting character limits.
Also, check out r/21CharactersAndNoMore, and r/21CharacterLimitation for other examples of subreddit limitations!